By setting this target property, the target is configured to build with C++/CLI support.
The Visual Studio generator defines the clr parameter depending on the value of COMMON_LANGUAGE_RUNTIME:
Supported values: "", "pure", "safe"
This property is only evaluated Visual Studio Generators for VS 2010 and above.
To be able to build managed C++ targets with VS 2017 and above the component C++/CLI support must be installed, which may not be done by default.
See also IMPORTED_COMMON_LANGUAGE_RUNTIME